NiTiHf high temperature shape memory alloys (HTSMAs) are being used in an ever-growing array of applications, specifically in the aerospace and automotive industries. One of the difficulties facing further implementation is ensuring the actuation fatigue lifetime is sufficiently long as to prevent the HTSMA components from being a limiting factor to the mean time between failures of a system. Another potential problem for widespread use is the deterioration of actuation stroke during lifetime, which can be problematic when attempting to have a high-fidelity repeatable design. One way of solving these issues is to optimize the microstructure through careful control of composition, processing, and heat treatments. Current research shows composition of large-scale productions is incredibly difficult to control, and small deviations in composition (~0.1 at.% Ni) can result in changes in transformation temperature by 50?C or more. Four NiTiHf compositions were investigated. The initial goal to simply extend the actuation fatigue lifetime and provide a stable actuation response morphed into determining material factors that influence the actuation response of partially cycled samples.

The use of superelastic Nitinol in implants continues to grow as physicians, scientists, and engineers design more novel medical devices to utilize its unique characteristics. As many of these devices are expected to be long-term implants, it becomes critically important to increase our understanding of Nitinol fatigue mechanisms beyond 10 7 cycles. In this study, the fatigue behavior of Nitinol wire in rotary bend testing was characterized by experimental methods and computational modeling. Fractures occurred in high strain regions as predicted by computational modeling. Furthermore, fractures beyond 10 7 or 10 8 cycles were observed and seem to have been initiated by nonmetallic inclusions.

Typical S/N type testing to determine the fatigue limit of Nitinol is done over a range of strain amplitudes; however, each specimen sees only a single peak strain amplitude during cycling. The effect of variable loading on Nitinol is therefore not understood. The purpose of this study was to evaluate any potential cumulative fatigue effect of combining low strain amplitude cycles with high strain amplitudes cycles on Nitinol wire apex specimens. A series of fatigue tests were performed to evaluate the fatigue response of Nitinol to variable loading. The results demonstrated that fatigue cycles at lower strain amplitudes can limit the number of higher amplitude cycles to failure in a variable loading scenario. However, the results also indicate that a small number of higher amplitude cycles can dominate the fatigue damage; almost all fractures occurred shortly after completing a section of higher amplitude cycles.

The temperature difference between active austenite finish temperature, A f , and the intended operating temperature in the range of 3.2 °C to 20.8 °C. has been reported to have an influence on the fatigue lifetime of a pseudoelastic shape-memory device. The negative effect on fatigue life increases with the temperature difference between active A f and, in case of a biomedical device, 37 °C body temperature. In this study, samples were prepared and processed in a manner to replicate aspects of the complex manufacturing process, device design, and geometry of state-of-the-art stents, and physiological loading conditions. Following explantation from the mock vessels after fatigue testing, the stents were inspected using optical microscopy to detect and document the location and number of strut fractures. The fatigue results were compared and assessed for statistical significance between the groups with various active A f temperatures. The variations in the heat treatments, as part of the manufacturing process, resulted in three distinct groups of samples with varying target active A f temperatures. These variances corresponded to differences in fatigue damage.

The effects of inclusions within a Nitinol alloy on the fatigue performance has been largely accepted by the Nitinol community. With significant reduction in size and density of non-metallic inclusions, the statistical probability of an inclusion being at the region of high stress and facilitating fatigue failure is much reduced. A new generation of commercial scale, ultra-low inclusion Nitinol material has been developed. The purpose of this study was to characterize this new alloy and demonstrate the improved fatigue performance. It was shown that a significant improvement fatigue performance can be realized with the new commercial scale ultra-low inclusion NiTi alloy. The results further suggest that inclusion size and density play a critical role in fatigue performance.